Two major aspects of the generation of B cell diversity occurring at different stages in cell differentiation will be analyzed in mice. The first experiments pertain to the biochemical nature of IGM produced by pre-B cells, the earliest recognizable cells of the B cell axis which contain IgM, detectable by immunofluorescence, in their cytoplasm, but not on their surface. First, the molecular size and the specific carbohydrate content of this IgM will be determined. Then, the ontogeny of clonal diversity among pre-B cells will be examined by 1. sequential analysis of the isoelectric patterns of the IgM by isolelectric focusing and 2. direct examination at a single pre-B cell level for the acquisition of different antibody specificities by using fluoresceinated antibodies to defined idiotypes. Anti-idiotypic antibodies will similarly be used to outline the ontogeny of clonal diversity among sIg plus B lymphocytes. A second set of experiments will be directed toward elucidation of the generation of isotypic diversity among surface immunoglobulin positive lymphocytes. For this, cellular aspects of LPS-mediated induction of IgG and IgA expression by IgM-bearing precursors will be studied in vitro, employing a variety of specific metabolic inhibitors. The regulatory mechanisms, including functions of T cells and macrophages, involved in development of plasma cells restricted to synthesis of one immunoglobulin isotype will also be determined.